The present invention relates to radio cellular mobile communications and relates to a downlink beamforming approach for frequency division duplex cellular systems. In particular, but not necessarily so restricted, the present invention relates to a downlink beamforming approach for code division multiple access (CDMA) systems.
To cope with the increasing demand for cellular mobile communications it is necessary to find ways to increase system capacity on the downlink whilst avoiding system complexity.
The cellular mobile communications IS-95 standard describes the use of direct sequence code division multiple access (CDMA) techniques. In such systems, each user is allocated a distinct pseudo-noise (PN) code. The signal from each user is multiplied by a respective code before transmission to the base station. All users transmit using the same radio frequency carrier. The signals from different users will arrive asynchronously due to their different locations and signals from each user may arrive asynchronously due to multipath propagation.
FIG. 1 is an algebraic representation of a CDMA communications link. The vector d contains N consecutive binary data symbols for P users. When these symbols are transmitted, they are subject to multipath distortion. This causes the receiver to observe J versions of each transmitted symbol, which arrive at different times. This effect is defined mathematically by two matrices.
Multiplying d by the matrix T repeats each symbol J times. The size NPJxc3x97NPJ matrix A is diagonal. Its diagonal elements are the positive square roots of the received multipath fading powers for the NP signals received on J paths. This results in the received signal being characterised as the product ATd. The size NPJxc3x97NPJ matrix (R/L)D represents the combined effects of beamforming and of pseudo-noise coding and decoding, where L is the CDMA processing gain. The size NPJxc3x97NPJ matrix R is Hermitian [Horn92, p169] (R. A. Horn and C. R. Johnson, xe2x80x9cMatrix Analysisxe2x80x9d, Cambridge University Press, Cambridge (UK), 1992.) The size NPJxc3x97NPJ matrix D is diagonal [Horn92, p23]. The quantity y=(R/L)DATd+z/L represents the processed received signal plus background noise (the size NPJ vector z/L).
Antenna array techniques have been proposed recently for improving the capacity of CDMA cellular systems as is described in xe2x80x9cSmart antenna arrays for CDMA Systemsxe2x80x9d IEEE Personal Communications Magazine, Vol 3(5) pp 16-25, October 1996. Antenna arrays are readily deployed at the base station on the uplink (mobile-to-base station link), as the uplink channel can be estimated from the received waveforms. Using antenna arrays to retransmit on the downlink in a frequency division duplex (FDD) system is more difficult, as channel estimates are not directly available to the base station. However, in order to increase CDMA system capacity, it is important to provide performance improvements on both radio links, although the downlink has not been the subject of much interest with respect to antenna arrays.
Thus, several approaches to downlink beamforming have been proposed. One approach to downlink beamforming is to use channel estimates for each user from the uplink to select beam patterns and impulse responses to transmit on the downlink. Another approach includes the use of DOA algorithms, although, the length of the multipath channel increases, so that the mobile may require more RAKE fingers to track all the multipath components.
In a frequency non-selective channel, neither a maximum SNR method (to be described below) nor the constrained downlink beamforming technique (also to be described below) will provide diversity at the mobile.
In an environment where frequency non-selective channels are common, a transmitter diversity scheme, such as phase sweeping or multipath diversity may be required to ensure diversity at the mobile (e.g. G. W. Wornell and M. D Trott, xe2x80x9cEfficient Signal Processing Techniques for Exploiting Transmit Antenna Diversity on Fading Channelsxe2x80x9d, IEEE Trans Sig Proc, January 1997, Vol 45(1), pp 191-205).
The present invention seeks to provide a simple to implement base station receiver structure which possesses improved symbol detection characteristics.
In accordance with a first aspect of the invention, there is provided a method of operating a FDD radio communications system wherein a base station has a constrained beamformer comprising at least two transmitting and receiving antennas, each with L channel taps and is operable to maximise diversity gain at the mobile, the steps comprising:
estimating the signal power for each tap on the uplink as:
xcex1i=(wHH(l)w)/(wHw)G
whereby to perform a simple diversity check for the maximum SNR weight vector solution w;
determining how the transmit power is spread;
wherein, in the event that the transmit power is concentrated in one tap, such that significant diversity gain is lost, an alternative beamformer solution w is determined whereby equal gain is provided in the directions of all L channel taps. The total SNR of all the multipath components received at the mobile will be less than for the maximum SNR algorithm, but the diversity gain should outweigh this loss for (nearly) orthogonal channel taps. The transmitter is restricted to using a single set of transmitter weights.
Preferably the L matrices G(l) are subject to eigenvalue decompositions to determine the L corresponding principal eigenvectors vl. The transmitter is restricted to using a single set of transmitter weights.
The weight vector solutions can be considered as determined by the following equation:
w=V(VHV)xe2x88x921f
wherein the lth column of the size Lxc3x97M matrix V is the eigenvector vl; and, f is the size Lxc3x971 constraint vector.
The weight vector solutions can be considered as determined by the following equation:
w=Vf
wherein the lth column of the size Lxc3x97M matrix V is the eigenvector vl; and, f is the size Lxc3x971 constraint vector.
The performance of an uplink fixed weight beamformer is determined from the correlation matrix of the fadings of the uplink sampled impulse response.